Typically, there have been used metal materials excellent in electroconductivity, as signal electrodes of a display device such as a liquid crystal display (LCD), an electroluminescence device (ELD), and a plasma display (PDP), and as electrodes of various circuit boards, such that copper, aluminum, silver, nickel, and the like are utilized depending on usages.
Here, as the conventional methods for forming metal electrodes, there have been widely used physical techniques such as a vacuum deposition method, and a sputtering method. These methods are capable of forming uniform metal films excellent in electroconductivity on substrates.
However, film formation apparatus to be used for the methods are based on vacuum vessels to become expensive, and require that the production apparatus are to be vacuumed for each film formation on a substrate, thereby exhibiting problems of production cost and mass-productivity. Further, it is required to form a resist and to conduct etching thereto so as to work a metal film into a predetermined pattern, which leads to an increased number of processes, thereby problematically lowering the productivity and leading to a higher cost.
To deal with the above-described problems, there have been proposed production methods (hereinafter called “coating methods” as the case may be) each configured to adopt a coating liquid for electroconductive film formation comprising a solvent containing silver or copper nanoparticles dispersed therein, in patent documents 1 to 3, respectively. According to such methods, there are formed electroconductive films such as made of silver, copper, and the like, by a simple production process including coating, drying, and calcining of a coating liquid for electroconductive film formation onto and on a substrate. This has been carried out, by successfully utilizing such a tendency that fine particles of silver, copper, and the like are fusion bonded to one another even at low temperatures by virtue of effects of nanoparticle sizes of them.
Although the coating methods can be applied to nanoparticles susceptible to fusion bonding, there is deteriorated a quality of an obtained film (i.e., the film is porous) in case of a metal insusceptible to fusion bonding such as nickel particles, so that the coating methods have not been put into a practical use except for usage where calcining is conducted at a high-temperature of about 1,000° C. such as in case of a multilayer ceramic capacitor. Further, since nickel is ferromagnetic, nickel fine particles tend to aggregate in a coating liquid when the coating liquid is low in viscosity, thereby also exhibiting a problem of stability of the coating liquid.
Thus, as a coating liquid which has improved the problems, there has been proposed a nickel film formation paste which adopts a molecular nickel source obtained by dissolving nickel formate in monoethanol amine (2-amino ethanol) (boiling point: 171° C.) as described in a patent document 4, for example. Further, there has been proposed a nickel film formation paste in a patent document 5, which is obtained by dissolving organic nickel such as nickel acetate in a glycol based solvent. According to these methods, the pastes are printed by screen printing, for example, and fired at temperatures at about 400° C. in a nitrogen gas to obtain nickel electroconductive films, respectively.
Incidentally, extensive researches have been recently conducted for inkjet printing as a method for carrying out coating and forming of a fine pattern with an excellent resolution upon formation of an electroconductive film based on a coating method, and coating liquids therefor have been thus sought for, which are excellent in ink jetting ability, which are excellent in film-forming ability, which are free of occurrence of defects such as cising (phenomenon where a coating liquid is decreased to a size smaller than a printed pattern, due to defective wettability of the coating liquid) and bleeding (phenomenon where a coating liquid is spread to a size wider than a printed pattern, due to excessive wettability of the coating liquid), and which are excellent in film characteristics such as electroconductivity.
However, screen printing is assumed and the coating liquids are each high in viscosity in case of the nickel film formation paste of the patent document 4 obtained by dissolving formic acid in monoethanol amine and the nickel film formation paste of the patent document 5 obtained by dissolving organic nickel such as nickel acetate in a glycol based solvent, so that these pastes fully fail to conform to a viscosity of as low as about 5 to 20 mPa·s assumed to be optimum for ink jetting ability in case of inkjet printing, resulting in failure of application of the pastes to inkjet printing.
Further, even if the patent document 4 is altered to prepare a nickel film formation ink having a low viscosity by adopting nickel formate and monoethanol amine, such an ink will be poor in film-forming ability and unsatisfactory in densification degree of a film, thereby failing to obtain a nickel electroconductive film which possesses sufficient film characteristics (combined possession of all of appropriate resistance value, film strength, film uniformity, surface roughness, and the like). Moreover, monoethanol amine is a deleterious substance such that the coating liquid containing monoethanol amine in excess of 20% is also classified as a deleterious substance, thereby also exhibiting a defect of inferior handling ability.
Here, in case of preparation of a nickel film formation ink having a low viscosity by nickel formate and an amine based solvent, the amine based solvent is alkaline, thereby bringing about a possibility that a substrate to which the ink is applied is deteriorated depending on the nature of the substrate because the amine based solvent acts as alkali. Examples of substrates susceptible to degradation by alkali include silicon oxide based ones. Thus, depending on substrates to be adopted, it has been additionally required to decrease the blending amount of an amine based solvent as less as possible in a nickel film formation ink.
Further, there has been desired film formation at lower temperatures (such as about 200 to 250° C.), because achievement thereof enables a widened applicability of a nickel film formation ink in case of forming a nickel film by coating the nickel film formation ink onto a substrate.    Patent document 1: JP-A-2002-334618    Patent document 2: JP-A-2002-338850    Patent document 3: JP-A-2003-103158    Patent document 4: JP-A-2005-026479    Patent document 5: JP-A-2004-265826